1. Field of the Invention
The invention relates to a water-spreading plastic material which is useful as a roofing, celling or glazing material.
2. Description of the Prior Art
Water-spreading plastics have the characteristic that water getting thereon will not be drawn up into separate droplets but rather will be spread so as to coalesce into a cohesive layer. This results in improved light reflection from the water-moistened surface and, in the case of transparent plastics, improved light transmission. Additionally, it is more difficult for water to drop from the underside of such plastic elements. These characteristics are particularly desirable for glazing materials for greenhouses, indoor swimming pools and similar humid structures. With respect to these materials, it is desirable that condensation forming thereon should not fall off in droplets, but rather should flow downwardly, following the shape of the material in a coherent layer, or at least flow in cohesive streams.
Numerous attempts have been made to produce coatings to protect against condensation which might collect on a plastic surface. One such type are those made of crosslinked hydrophilic polymer. For instance, according to DE-OS No. 21 61 645, a coating of this type is produced from a mixed polymer of alkyl ester, hydroxyalkyl ester and quarternary aminoalkyl ester of acrylic or methacrylic acid and methylol ethers of methacrylic amide as linking agents. Such coating are rather interesting since they initially are not water spreading in nature but become so as they absorb water and thereby swell, gradually changing into a water-spreading coating. As a result of the swelling, however, the coating is soft and susceptible to mechanical damage.
In order to achieve a greater mechanical durability, coatings were developed with hydrophilic inorganic components in a hydrophilic binding agent. In JP-A No. 76 81 877, PVC (polyvinylchloride) or polymethyl methacrylate films (or sheets) are covered with a coating of colloidal aluminum oxide as a hydrophilic, solid content material and polyvinyl alcohol and ammonium polyacrylate as binding agents. However, in its water-swollen condition, this coating is also mechanically sensitive.
It has also been attempted to place crosslinking agents into the plastic material itself. Water-spreading coverings for greenhouses and similar humid structures are manufactured, according to DE-PS No. 2 017 002, from a plastic containing a surface active agent, such as polyalkyl glycol. The effect of this additive is limited, and in addition, the weather resistance of the plastic is reduced.
In JP-A No. 76 06 193, polymethyl methacrylate plates having a coating of 95 parts colloidal silicon dioxide and 5 parts of a dispersion of a hydrophobic acrylic resin is suggested as a glazing material for greenhouses. The adhesion of this coating, however, is completely unsatisfactory, particularly when moistened.
A better adhesion of a condensation-protective coating on plastics is achieved according to EP-A No. 51 405 with a covering composed of two layers. Both of these layers contain colloidal silicon dioxide, a partially hydrolyzed silicone and polyvinyl alcohol as a binding agent. The ratio of silicon to carbon is greater in the lower layer than in the outer layer.
The Japanese reference No. 83/63 729 discloses a two-stage manufacturing method for a water-spreading coating on plastics which is not susceptible to being washed off. A solution of a non-water-soluble, anion-active polyelectrolyte, dissolved in an organic solvent, is applied in a thin layer as a base coat. Examples are mixed polymers of methyl and butyl methacrylate and methacrylic acid or of styrene and maleic acid. An aqueous, positively electrically charged, inorganic colloidal sol is applied to the base coat and is adsorbed as a result of the reciprocal action with the anion-active base coat. One example of this type of colloidal sol is a cation-active clay sol sold in commerce by Nissan Chemical Industries Ltd. under the name "Aluminasol 100." Although this coating exhibits a good water-spreading effect, it has the disadvantage of being highly sensitive to mechanical damage. In the wet-scouring test according to DIN 53 778, linear areas of detachment (separation) appeared after just 10 scouring strokes and after 50 scouring strokes the coating was completely separated. For the stresses greenhouse glazing is subjected to during erection and operation, this is completely unsatisfactory.
The great number of condensation-protective covering systems for plastics makes evident that the strong need for a coating that is useable in practice has not been sufficiently provided for by the coverings developed to date. Strongly hydrophilic covering materials generally achieve a good waterspreading effect, but in swelled condition they are too soft. If one attempts to combat this disadvantage with stronger crosslinking or reduced hydrophilic properties, then the water-spreading capability is lost together with the mechanical sensitivity. Silicon dioxide and various other metallic oxides combine the advantage of greater hardness and good water wettability without swelling. But these oxides have no adhesion to the surface of a plastic whatsoever. Adhesive base layers improve the adhesion of the oxides, but even they remain inadequate to the practical requirements.
To the degree that binding agents are used to anchor the oxides to the surface of the plastic, the wettability of the oxides is reduced and the disadvantages of binding agents appear: namely mechanical sensitivity in the case of hydrophilic binding agents and insufficient water-spreading in the case of hydrophobic binding agents.
It is indeed known to apply coverings of colloidal silicic acid onto a substrate having a textured surface, such as texturized fibers or paper, for example, to reduce surface sliding properties. In these cases, the silicic acid is anchored onto the surface of the substrates. Such silicic acid covering has been used as an anti-cling agent on a plastic film, whereby permanent adhesion is hindered. Thus, the rolled end of the plastic is prevented from clinging until it is removed from its supply roll. Anti-clinging property is provided by partially destroying the silicic acid coating when separating adjacent layers of the plastic.